Conventional ultrasound scanners create two-dimensional B-mode images of tissue in which the brightness of a pixel is based on intensity of the echo return. Alternatively, in a color flow imaging mode, movement of fluid (e.g., blood) or tissue can be imaged. Measurement of blood flow in the heart and vessels using the Doppler effect is well known. The phase shift of backscattered ultrasound waves may be used to measure velocity of the backscatterers from tissue or blood. The Doppler shift may be displayed using different colors to represent speed and direction of flow. In power Doppler imaging, the returned Doppler signal power is displayed. Although the following discussion refers predominantly to B-mode imaging for the sake of brevity, the present invention applies to any mode of ultrasound imaging.
Two-dimensional ultrasound images are often difficult to interpret due to inability of the observer to visualize the two-dimensional representation of the anatomy being scanned. In addition, it may not be possible to acquire the precise view needed to make a diagnosis due to probe geometry or poor access to the area of interest. However, if the ultrasound probe is swept over an area of interest and two-dimensional images are accumulated to form a three-dimensional data volume, the anatomy becomes much easier to visualize for both the trained and untrained observer. Also, views which cannot be acquired due to probe geometry or poor access to the area of interest can be reconstructed from the three-dimensional data volume by constructing slices through the volume at the angle that is otherwise difficult to obtain.
In order to generate three-dimensional images, the imaging system computer can transform a source data volume retrieved from memory into an imaging plane data set. The successive transformations may involve a variety of projection techniques such as maximum, minimum, composite, surface or averaged projections made at angular increments, e.g., at 10.degree. intervals, within a range of angles, e.g., +90.degree. to -90.degree.. Each pixel in the projected image includes the transformed data derived by projection onto a given image plane.
In free-hand three-dimensional ultrasound scans, a transducer array (1D to 1.5D) is translated in the elevation direction to acquire a set of image planes through the anatomy of interest. These images can be stored in memory and later retrieved by the system computer for three-dimensional reconstruction. If the spacings between image frames are known, then the three-dimensional volume can be reconstructed with the correct aspect ratio between the out-of-plane and scan plane dimensions. If, however, the estimates of the inter-slice spacing are poor, significant geometric distortion of the three-dimensional object can result.
A conventional ultrasound imaging system collects B-mode, color flow mode and power Doppler mode data in a cine memory on a continuous basis. As the probe is swept over an area of the anatomy, using either a free-hand scanning technique or a mechanical probe mover, a three-dimensional volume is stored in the cine memory. The distance the probe was translated may be determined by any one of a number of techniques. The user can provide an estimate of the distance swept. Alternatively, if the probe is moved at a constant rate by a probe mover, the distance can easily be determined. Attachment of a position sensor to the probe to determine the position of each slice os amptjer a;termatove/. Markers on the anatomy or within the data could also provide the required position information. Yet another technique is to estimate the scan plane displacements directly from the degree of speckle decorrelation between successive image frames. Once the data volume has been acquired, the central processing unit can provide three-dimensional projections of the data as well as arbitrary slices through the data volume.
It is often desirable to form a three-dimensional projection of the surface of an internal structure during diagnostic medical imaging. For instance, in obstetrics, it is often desirable to scan parts of the fetal anatomy (hands, feet, or face) to look for possible fetal defects. Techniques have been developed which produce a single three-dimensional projection of fetal anatomy from a fixed position. These techniques require that the baby's face be oriented so that it is looking up at the transducer and most often use special positioning devices such as a motorized probe mover. A range gate is often used to eliminate unwanted anatomy from the data volume to be rendered. In addition, simplifying assumptions are made in the compositing algorithm to speed up rendering. While this technique appears to produce reasonable results, it has several drawbacks.
In practice, it is unrealistic to require that the baby being imaged be in a fixed orientation, so any technique with this requirement will have limited application. Further, a single view is often not enough to make a definitive diagnosis of a defect. Thus there is a need for a technique which does not require either a special probe or that the object being imaged be in any particular orientation and which allows the object to be viewed from any angle.